Segmented Photodiode Pixel for Dental Imaging Noise Reduction
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Solution Overview
Problem
Solid-state image pickup devices for dental applications face challenges with large photodiode junction capacitance leading to high noise and the need for increased X-ray exposure, which is undesirable for medical imaging, especially in dental 3D-CT where large photosensitive regions are required for low X-ray exposure and high S/N ratio.
Innovation Solution
A solid-state image pickup device with a two-dimensional arrangement of pixels, each containing multiple photodiodes with square-shaped photosensitive regions, a signal readout section, A/D converting section, and an adding section that sums digital values from multiple photodiodes to improve S/N ratio and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the photosensitive region of each pixel is made large to reduce X-ray exposure and improve sensitivity, then the S/N ratio improves, but the photodiode junction capacitance increases leading to increased noise
Solution Approach 1:
Each pixel is divided into multiple photodiodes (e.g., four photodiodes) instead of using a single large photodiode. This segmentation allows the photosensitive area to be maintained while reducing the junction capacitance of each individual photodiode, thereby reducing noise while preserving sensitivity
Solution Approach 2:
The signals from multiple photodiodes within each pixel are combined through adding sections that sum the digital values. This merging of signals from segmented photodiodes achieves the same effect as a large single photodiode (improved sensitivity) while avoiding the penalty of high junction capacitance (noise reduction)
2Use of energy by moving object
If the photosensitive region of each pixel is made large to improve sensitivity, then fewer X-rays are needed for imaging, but the amount of X-ray exposure must still be increased to maintain S/N ratio
Solution Approach 1:
By segmenting each pixel into multiple smaller photodiodes, the junction capacitance is reduced which directly reduces noise. This allows maintaining or improving S/N ratio without needing to increase X-ray exposure, as the reduced capacitance enables better signal quality at lower exposure levels
Solution Approach 2:
The invention changes the structural parameter of the photodiode from a single large element to multiple smaller elements. This parameter change (division into multiple photodiodes) fundamentally alters the capacitance characteristics, enabling operation at lower X-ray exposure while maintaining acceptable S/N ratio
3Device complexity
If each pixel contains only one photodiode with large area, then the structure is simple, but the junction capacitance is large causing high noise
Solution Approach 1:
The photodiode structure within each pixel is segmented into multiple smaller photodiodes. While this increases the number of components slightly, it dramatically reduces noise through lower junction capacitance. The added complexity is minimal compared to the significant noise reduction benefit
Solution Approach 2:
Instead of increasing photodiode area in one dimension (which increases capacitance), the invention distributes the photosensitive area across multiple photodiodes in a two-dimensional arrangement within each pixel. This dimensional approach maintains total sensitivity while reducing individual capacitance values
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The device achieves an excellent S/N ratio and reduced X-ray exposure by halving the noise component and extending the dynamic range, making it suitable for dental applications with improved imaging capabilities.
Implementation Method 1
a plurality of pixels each including a plurality of photodiodes PD1 and PD2... an electric charge generated in the plurality of respective photodiodes
Data Source
AI summary
For a solid-state image pickup device 1, a plurality of pixels are two dimensionally arranged in an imaging region 10, and two photodiodes PD1 and PD2 are included in each pixel Pm,n. An electric charge generated in the respective photodiodes PD1 and PD2 is input to a signal readout section 20, and a voltage according to an electric charge amount thereof is output from the signal output section 20. The voltage output from the signal readout section 20 is input to an A/D converting section 40, and a digital value according to the input voltage is output from the A/D converting section 40. In an adding section 50, a sum of digital values to be output from the A/D converting section 40 according to the amount of electric charge generated, for each pixel Pm,n of the imaging region 10, in the two respective photodiodes PD1 and PD2 included in the pixel is operated, and a digital value being a sum value thereof is output.